Here's a simple script to migrate Freeagent invoice data -> Freshbooks. Putting this out there in case it helps someone else.

freshbooks_importer.rb

# gem install ruby-freshbooks
# https://github.com/elucid/ruby-freshbooks
#
# * Limitations *
# - Freshbooks Invoice #'s can only be 10 characters
# - requires creating clients ahead of time (though this would be easy for someone to add)
# - client names must match exactly
#
# * Usage *
# require 'freshbooks_importer'
# freshbooks_importer = FreshbooksImporter.new
# freshbooks_importer.import_invoices

require 'ruby-freshbooks'
require 'csv'

class FreshbooksImporter
  attr_accessor :connection
  attr_accessor :current_invoice
  attr_accessor :clients

  def initialize
    self.connection = FreshBooks::Client.new('your-url.freshbooks.com', FRESHBOOKS_API_KEY)
    $invoices = 1
  end

  def import_invoices(csv='invoices.csv')
    CSV.foreach(csv, headers: true) do |row|
      if !row['Contact'].nil?
        create_invoice row
      elsif !current_invoice.nil?
        append_line_item_to_current_invoice row
      else
        puts 'no current invoice for line item.'
      end
    end

    # save the final invoice
    save_current_invoice
  end

  private

  def create_invoice(row)
    save_current_invoice

    client = freshbooks_client(row['Contact'])
    build_invoice_for_client(client, row)
  end

  def build_invoice_for_client(client, row)
    if !client.nil?
      puts "building invoice for client #{client['organization']}..."
      self.current_invoice = Invoice.new(connection, row)
      current_invoice.freshbooks_client_id = client['client_id']
    end
  end

  def save_current_invoice
    if !current_invoice.nil?
      current_invoice.save
    end
  end

  def append_line_item_to_current_invoice(row)
    line_item = LineItem.new row

    current_invoice.line_items << line_item
  end

  def freshbooks_client(client_name)
    client = freshbooks_client_list.detect { |c| c['organization'] == client_name  }
    if !client.nil?
      client
    else
      puts "client #{client_name} not found!"
      nil
    end
  end

  def freshbooks_client_list
    self.clients ||= fetch_freshbooks_client_list
  end

  def fetch_freshbooks_client_list
    list = connection.client.list
    list['clients']['client']
  end

end

class Invoice
  attr_accessor :contact, :reference, :date, :status, :net_amount, :line_items, :freshbooks_client_id, :connection

  def initialize(connection, row)
    super()
    self.connection = connection
    set_attributes(row)
    self.line_items = []
  end

  def save
    puts "ready to create with: #{attributes_for_freshbooks.inspect}"
    new_invoice = connection.invoice.create(invoice: attributes_for_freshbooks)

    if new_invoice && !new_invoice['id'].nil?
      $invoices += 1
      puts "invoice #{id} created!"
    end
  end

  def attributes_for_freshbooks
    {
      client_id: freshbooks_client_id,
      number: shorten_invoice_number(reference),
      status: status.downcase,
      date: freshbooks_date,
      lines: line_items.map(&:attributes_for_freshbooks)
    }
  end

  private

  def set_attributes(row)
    self.contact    = row['Contact']
    self.reference  = row['Reference']
    self.date       = row['Date']
    self.net_amount = row['Net Amount']
    self.status     = row['Status']
  end

  def shorten_invoice_number(invoice_number)
    without_digits = invoice_number.sub(/(\d+)/,'').strip
    number = $1
    abbreviation = without_digits.split(/\s+|_/).map{|s| s[0]}.join
    "#{abbreviation}#{number || $invoices}"
  end

  def freshbooks_date
    # convert free agent date -> standard
    Date.strptime(date, '%d %B %Y').to_s
  end
end

class LineItem
  attr_accessor :type, :quantity, :price, :description, :subtotal

  def initialize(row)
    super()
    set_attributes(row)
  end

  def attributes_for_freshbooks
    {
      line: {
        description: description,
        unit_cost: price,
        quantity: quantity,
        type: freshbooks_type
      }
    }
  end

  private

  def set_attributes(row)
    self.type        = row['Item Type']
    self.quantity    = row['Quantity']
    self.price       = row['Price']
    self.description = row['Description']
    self.subtotal    = row['Subtotal']
  end

  def freshbooks_type
    if type == 'Hours'
      'Time'
    else
      'Item'
    end
  end
end